A cold well is an insulated, refrigerated basin built into a counter or serving table. GN pans of food sit in it, and cooling coils in the walls or base, or a fan-forced system, keep the pans cold. Some designs use a bed of ice instead of refrigeration, but refrigerated wells hold temperature more consistently and do not need ice replenished.
Cold wells are meant to hold food that is already chilled, not to cool warm food down. Pans should go in cold, be kept shallow and not heaped above the rim, and be swapped regularly rather than topped up. Sneeze guards or covers protect the food from customers and dust, which matters in open buffets.
A corporate cafeteria in Noida might run a salad and dessert line with two cold wells: one for cut salads, raita and chutneys, and one for desserts like fruit custard and phirni. Pans are replaced from a reach-in every hour during lunch to keep everything within safe temperature.